Book/Dissertation / PhD Thesis

FZJ-2017-03774

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Spin-wave excitations and electron-magnonscattering in elementary ferromagnets from $\textit{ab initio}4 many-body perturbation theory

Müller, M. C. T. D. (Corresponding author)FZJ*

2017
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag Jülich
ISBN: 978-3-95806-242-9

Please use a persistent id in citations: http://hdl.handle.net/2128/14661

Abstract: In this thesis, an $\textit{ab initio}$ theoretical framework for the investigation of spin excitations and the electron-magnon scattering is developed within many-body perturbation theory and implemented in the full-potential linearized augmented-plane-wave method. The spin excitations, including single-particle Stoner excitations and collective spin waves, are accessible through the magnetic response function, which is obtained by the solution of a Bethe-Salpeter equation employing four-point functions. These four-point functions are represented in a Wannier-function basis, which allows to exploit the short-range behavior of the screened interaction in metallic systems by truncating the matrices in real space. The spin excitation spectrum of ferromagnetic materials contains an acoustic magnon mode whose energy, in the absence of spin-orbit coupling, vanishes in the long-wavelength limit as a consequence of the spontaneously broken spin-rotation symmetry in these materials according to the Goldstone theorem. However, in numerical realizations of the magnetic response function the acoustic magnon mode exhibits a small but finite gap in the Goldstone-mode limit. We investigate this violation of the Goldstone theorem and present anapproach that implements the magnetic response function employing the properly renormalized Green function instead of the Kohn-Sham one. This much more expensive approach shows a substantial reduction of the gap error. In addition, we discuss a correction scheme motivated by the one-band Hubbard model that cures the fundamental inconsistency of using the Kohn-Sham Green function by adjusting the exchange splitting. We present corrected magnon spectra for the elementary ferromagnets iron, cobalt, and nickel. We then employ the T-matrix approach for the description of the electron-magnon interaction within the GT approximation, which can be combined with the GW approximation without the need of double-counting corrections. The multiple-scattering T matrix is part of the four-point magnetic response function and describes the correlated propagation of electron-hole pairs with opposite spins from which the collective spin excitations arise. We apply the GT approximation to Fe, Co, and Ni and present renormalized spectral functions. The GT approximation leads to a pronounced spin-dependent lifetime broadening of the quasiparticle states to the extent that the quasiparticle character is virtually lost in certain energy regions. In iron, the spectral functions exhibit an additional quasiparticle peak indicating the emergence of a new quasiparticle. We discuss the features of this quasiparticle state that forms out of a superposition of single-particle and magnon excitations. In addition, we find kink structures in the quasiparticle dispersion of free-electron-like bands of cobalt and nickel.

Note: RWTH Aachen, Diss., 2017

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Contributing Institute(s):

1. Quanten-Theorie der Materialien (PGI-1)
2. Quanten-Theorie der Materialien (IAS-1)
3. JARA-FIT (JARA-FIT)
4. JARA - HPC (JARA-HPC)

Research Program(s):

1. 142 - Controlling Spin-Based Phenomena (POF3-142) (POF3-142)
2. 143 - Controlling Configuration-Based Phenomena (POF3-143) (POF3-143)

Appears in the scientific report 2017

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